1. Technical Field:
The present application relates to a technique of extracting respiration. More specifically, the present application relates to a technique of extracting information concerning respiration, by measuring biological impedance with a plurality of electrodes which are worn on the body of a user.
2. Description of the Related Art:
In recent years, methods of electrically and mechanically measuring and recording the physical status of a user over long hours are becoming increasingly popular. Examples of basic electrical information representing the physical status of a user include the electroencephalogram (EEG), which is related to the brain, and the electrocardiogram (ECG), which is related to motions of the heart. Between these, an electrocardiogram may be acquired at a hospital as fundamental biological information (vital signs), for example. An electrocardiogram may also be acquired when a heart disease is suspected, by using a portable-type electrocardiograph called the Holter electrocardiograph. Use of the Holter electrocardiograph makes it possible to record an electrocardiogram at places other than a hospital, such as one's own home, over long hours, e.g., 24 hours. In recent years, the Holter electrocardiograph has been downsized, making it easier for a user to measure electrocardiograms.
Recording an electrocardiogram over long hours with a Holter electrocardiograph makes it possible to discover symptoms such as arrhythmia, which cannot be detected through short-time checkups at hospital. However, apart from the electrocardiogram, there are checkup items (clinical cases) which can only be exposed through long hours of checkup, e.g., sleep apnea syndrome. Sleep apnea syndrome is a disease of the respiratory system that is deeply related to arrhythmia.
Assessments of sleep apnea syndrome cannot be made with electrocardiograms alone, but also require information concerning respiration. Currently, a sleep apnea syndrome assessment requires an overnight polysomnography checkup, which simultaneously takes electrocardiogram, respiration, and electroencephalogram measurements. This checkup needs to be conducted during an overnight stay at hospital, which presents significant burdens on the hospital and on the patient. Therefore, mere suspicion of a disease possibility would not practically justify such a high-burden checkup.
If it were possible to obtain information concerning diseases of the respiratory system—or specifically, respiratory rate-related information—, as easily as if obtaining an electrocardiogram with a Holter electrocardiograph, it would further promote early disease detection and quickened diagnosis.
When measuring respiration in simple manners, the main choice so far has been a medical device called the pulse oximeter. This is a measuring device for examining arterial oxygen saturation. Arterial oxygen saturation is measured with a sensor worn at the fingertip, called a probe. This measuring device, which includes a red light source (LED), measures the oxygen content in the arteries inside a finger in real time by measuring light transmitted through the finger with the sensor as red light is emitted from the LED. Thus, when both electrocardiogram and respiration information is necessary, it has been necessary to wear the electrodes for an electrocardiograph on the thorax, and wear the probe of a pulse oximeter at the fingertip.
What has been done so far is to use a single apparatus to simultaneously acquire electrocardiogram and respiration information, and isolate respiration information from the acquired data by using electrocardiogram information. One approach is the impedance method. Under the impedance method, an electric current is flown in the user's body, and impedance changes associated with the electrocardiogram and respiration are measured with electrodes which are placed on the thorax. For example, the specification of Japanese Patent No. 3735774 discloses a method for removing the respiratory component from thoracic impedance.
FIG. 1A shows fundamental components of an electrocardiogram. In FIG. 1A, the Q, R, and S waves represent ventricular excitation.
FIG. 1B shows an example waveform which is obtained as an electrocardiogram. In the specification of Japanese Patent No. 3735774, an electrocardiographic component is estimated based on a linear combination model of cosine components and sine wave components of harmonics whose fundamental wave is the R-R interval of an electrocardiogram. This method is called the SFLC (Scaled Fourier Linear Combiner) method.
Yoshifumi Yasuda, et. al., “Modified thoracic impedance plethysmograph to monitor sleep apnea syndromes”, Sleep Medicine, Vol. 6, pp. 215-224 (2005) (hereinafter referred to as “Non-Patent Document 1”) discloses a technique which, by utilizing the method described in the specification of Japanese Patent No. 3735774, obtains a respiratory component by subtracting an electrocardiographic component which is estimated by the SFLC method from thoracic impedance. FIG. 2 shows the concept behind the conventionally-employed technique of extracting a respiratory component (c) by subtracting a component of electrocardiographic origin (b) from thoracic impedance (a). If there is a change in respiration, it is reflected in the waveform of the extracted respiratory component.
FIGS. 3A and 3B show impedance changes when changes occur in respiration. FIG. 3A shows impedance changes in the case where normal respiration is followed by a state of low respiration, and further followed by normal respiration. It can be seen that the amplitude itself decreases in the zone of low respiration. FIG. 3B shows impedance changes in the case where a state of obstructive apnea is created after normal respiration. It can be seen in this case that, in the zone of apnea, the amplitude of impedance is lost altogether. Thus, impedance changes not only reflect the respiratory rate, but also reflect the aspiration volume.
Another approach is the ECG (Electrocardiogram) method. Under the ECG method, potential changes based on an electrocardiogram and respiration are measured with electrodes which are placed on the thorax, without flowing an electric current. Ciara O'Brien, Conor Heneghan, “A comparison of algorithms for estimation of a respiratory signal from the surface electrocardiogram”, Computers in Biology and Medicine, Vol. 37, Issue 3, pp. 305-314 (2007) (herein referred to as “Non-Patent Document 2) discloses a technique of extracting a respiratory component by finding an envelope of R waves on the time axis.